2,2′-Homocoupled Azine N ,N′ -Dioxides or Azine N -Oxides: CDC- or SN Ar-Controlled Chemoselectivity

Recent advances on non-precious metal-catalyzed C-H functionalization of N-heteroarenes

N-Heteroarenes are widely used for numerous medicinal applications, lifesaving drugs and show utmost importance as intermediates in chemical synthesis. This feature article highlights the recent advances, from 2015 to August 2021, on sp² and sp³ C-H bond functionalization reactions of various N-heteroarenes catalyzed by non-precious transition metals (Mn, Co, Fe, Ni, etc.). The salient features of the report are: (i) the development of newer catalysis for Csp²-H activation of N-heteroarenes and categorized into alkylation, alkenylation, borylation, cyanation, and annulation reactions, (ii) recent advances on Csp³-H bond functionalization of N-heteroarenes considering newer approaches for alkylation as well as alkenylation processes, and (iii) synthetic applications and practical utility of the catalytic protocols utilized for late-stage drug development; (iv) scope for the development of newer catalytic protocols along with mechanistic studies and detail mechanistic findings of various important processes.

Synthesis of Nitrile-Functionalized Polydentate N-Heterocycles as Building Blocks for Covalent Triazine Frameworks

Covalent triazine frameworks (CTFs) based on polydentate ligands are highly promising supports to anchor catalytic metal complexes. The modular nature of CTFs allows to tailor the composition, structure, and function to its specific application. Access to a broad range of chelating building blocks is therefore essential. In this respect, we extended the current available set of CTF building blocks with new nitrile-functionalized N-heterocyclic ligands. This paper presents the synthesis of the six ligands which vary in the extent of the aromatic system and the denticity. The new building blocks may help in a rational design of enhanced support materials in catalysis.

Recent Advances in the Synthesis of 2,2′-Bipyridines and Their Derivatives

The sustained interest in the synthesis of new analogues of 2,2′-bipyridines is supported by the importance of compounds featuring bipyridine core in diverse areas of chemical, biomedical and materials research, which is relayed into the development of new approaches and the expansion of existing synthetic methods. This short review covers advances in the synthesis of 2,2′-bipyridines, including both the synthesis of compounds with a given substitution pattern and the development of new methods for assembling the bipyridine core. Special attention is directed toward the use of pyridine N-oxides and metal-free protocols to facilitate the formation of bipyridines. This short review focuses primarily on reports published in the last 5–6 years.

1 Introduction

2 Ullmann-Type Homocoupling Reactions

3 Cross-Coupling Reactions in the Synthesis of Bipyridines

4 Coupling Reactions Employing Pyridine N-Oxides

5 Other Methods for the Synthesis of 2,2′-Bipyridines

6 Conclusions and Outlook

The 2-Pyridyl Problem: Challenging Nucleophiles in Cross-Coupling Arylations

Azine-containing biaryls are ubiquitous scaffolds in many areas of chemistry, and efficient methods for their synthesis are continually desired. Pyridine rings are prominent amongst these motifs. Transition-metal-catalysed cross-coupling reactions have been widely used for their synthesis and functionalisation as they often provide a swift and tuneable route to related biaryl scaffolds. However, 2-pyridine organometallics are capricious coupling partners and 2-pyridyl boron reagents in particular are notorious for their instability and poor reactivity in Suzuki-Miyaura cross-coupling reactions. The synthesis of pyridine-containing biaryls is therefore limited, and methods for the formation of unsymmetrical 2,2'-bis-pyridines are scarce. This Review focuses on the methods developed for the challenging coupling of 2-pyridine nucleophiles with (hetero)aryl electrophiles, and ranges from traditional cross-coupling processes to alternative nucleophilic reagents and novel main group approaches.

1,1'-Biisoquinolines-Neglected Ligands in the Heterocyclic Diimine Family That Provoke Stereochemical Reflections

1,1′-Biisoquinolines are a class of bidentate nitrogen donor ligands in the heterocyclic diimine family. This review briefly discusses their properties and the key synthetic pathways available and then concentrates upon their coordination behaviour. The ligands are of interest as they exhibit the phenomenon of atropisomerism (hindered rotation about the C1–C1′ bond). A notation for depicting the stereochemistry in coordination compounds containing multiple stereogenic centers is developed. The consequences of the chirality within the ligand on the coordination behaviour is discussed in detail.

Efficient stereoselective synthesis of chiral 3,3′-dimethyl-(2,2′-bipyridine)-diol ligand and applications in FeII-catalysis

A seven step synthesis of a chiral 2,2′-bipyridinediol ligand with 3,3′-dimethyl substituents was achieved starting from commercially available materials.

Stereoselective Synthesis of Atropisomeric Bipyridine N,N'-Dioxides by Oxidative Coupling

Bipyridine N,N'-dioxide is a structural fragment found in many bioactive compounds. Furthermore, chiral analogues secured their place as powerful Lewis base catalysts. The scope of the existing methods for the synthesis of atropisomeric bipyridine N,N'-dioxides is limited. Herein, we present a practical, highly chemo- and stereoselective method for oxidative dimerization of chiral pyridine N-oxides using O₂ as a terminal oxidant. A series of 13 axially chiral bipyridine N,N'-dioxides were synthesized in up to 75% yield.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.